This invention relates to a process for producing dimethyl-2,6-naphthalene dicarboxylate (hereinunder referred to as 2,6-NDM) which is useful as a raw material for high quality polyester.
In the prior art, 2,6-naphthalene dicarboxylic acid (hereinafter referred to as 2,6-NDA) has been produced by the following methods:
(1) Processes for producing 2,6-NDA which comprise oxidizing 2,6-dimethyl naphthalene in the presence of a catalyst comprising a heavy metal and a bromine compound are disclosed in U.S. Pat. No. 3,856,855 and Japanese Patent Publication (Kokai) No. 34153/1973.
(2) A process for producing 2,6-NDA which comprises oxidizing 2,6-diisopropyl naphthalene in the presence of a catalyst comprising Co and Mn is disclosed in Japanese Patent Publication (Kokai) No. 89445/1985.
(3) Processes for producing 2,6-NDA which comprise oxidizing a 2-alkyl-6-acyl naphthalene in the presence of a catalyst containing Co and Mn or Co, Mn and Br are disclosed in Japanese Patent Publication (Kokai) Nos. 61946/1987 and 67048/1987 and U.S. Pat. No. 4,764,638.
A naphthalene ring is more oxidizable than a benzene ring. So where a naphthalene compound is used as a reactant, many by-products and condensates are likely to be formed due to ring decomposition. Therefore, the 2,6-NDA produced by the above-mentioned methods contains large amount of impurities. Generally speaking, after the 2,6-NDA containing impurities is converted to 2,6-NDM, the 2,6-NDM is purified.
In the prior art, 2,6-NDM has been purified by (i) vacuum distillation; (ii) vacuum distillation and recrystallization [refer to Japanese Patent Publication (Kokai) No. 111055/1975, U.S. Pat. No. 4,048,021, Japanese Patent Publication (Kokai) No. 116461/1975 and Japanese Patent Publication (Kokoku) No. 35697/1982]; and (iii) recrystallization method by using a solvent, such as chlorobenzene, xylene or methanol [Japanese Patent Publication (Kokoku) Nos. 9697/1971 and 40349/1973].
It was thought that the vacuum distillation method is the easiest. However, since the melting point of 2,6-NDM is 190.degree. C., it is difficult to carry out the distillation operation of 2,6-NDM early. That is, when 2,6-NDM is purified by the vacuum-distillation method, it is necessary to maintain the condenser at a temperature of more than 200.degree.-210.degree. C. in order to avoid solidification of 2,6-NDM in the condenser from occurring while the 2,6-NDM is being cooled in the condenser. When the distillation temperature is not so high when of purifying 2,6-NDM by vacuum distillation, the proportion of 2,6-NDM to be not condensed is increased. The 2,6-NDM which is not condensed in the condenser is condensed in a solidifying receiver which is maintained at a temperature less than the melting point of 2,6-NDM. When the proportion of 2,6-NDM to be condensed in the solidifying receiver becomes large, it becomes complicated to remove the 2,6-NDM from the solidifying receiver.
In addition, 2,6-NDM is likely to accumulate at the vacuum pump, etc. which results in the ability of the vacuum pump being lowered, so that the pipe is likely to become choked. It becomes necessary to raise the distillation temperature in order to avoid this trouble. However, when the distillation temperature rises, 2,6-NDM is likely to decompose or change in quality which results in a lowering in the quality of the product. In addition, highly viscous, high boiling point materials which are formed in the process adhere to the heat transfer surface of the heat exchanger. This is likely to lead to a decrease in heat transfer and cause local overheating.
As the 2,6-NDM distillate obtained by vacuum-distillation is not necessarily of a high quality, for example, in the invention of Japanese Patent Publication (Kokoku) No. 3057/1971 disclosing vacuum-distillation for purification of 2,6-NDM, an additive is added to the 2,6-NDM and distillation is effected to improve the acid value and color of 2,6-NDM.
2,6-NDM may be purified by recrystallization by using a solvent, such as chlorobenzene, etc. When the acid value and degree of coloration of crude 2,6-NDM are low, recrystallization by using the solvent can be easily carried out. However, when they are high, the recrystallization process must be repeated several times to purify the 2,6-NDM. The behavior of acidic materials and coloring materials is similar to that of 2,6-NDM, so 2,6-NDM cannot be completely purified by recrystallization. Therefore, in the case of purifying 2,6-NDM by recrystallization, an adsorbent is added to the 2,6-NDM during the recrystallizing operation, thereby removing the materials.
Japanese Patent Publication (Kokoku) No. 43731/1973 discloses a process for purifying 2,6-NDM which comprises adding an oxide or a hydroxide compound of an element belonging to Group I or II of the Periodic Table, an acid carbonate, a carbonate, an acid sulfate, an acid sulfite or an organic acid salt compound to crude 2,6-NDM to react with the acidic material contained in the 2,6-NDM, thereby depositing the salt in the solvent, and then carrying out thermal filtration to remove the salt. In this process, when the thermal filtration is not precisely carried out, the resulting 2,6-NDM is contaminated by a small amount of the additive and the salt which is dissolved in the solvent, and as a result, highly pure 2,6-NDM cannot be obtained.